1. Field of the Invention
This invention relates to electrical switching apparatus and, more particularly, to circuit breakers, such as, for example, arc fault circuit breakers.
2. Background Information
Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. In small circuit breakers, commonly referred to as miniature circuit breakers, used for residential and light commercial applications, such protection is typically provided by a thermal-magnetic trip device. This trip device includes a bimetal, which heats and bends in response to a persistent overcurrent condition. The bimetal, in turn, unlatches a spring powered operating mechanism, which opens the separable contacts of the circuit breaker to interrupt current flow in the protected power system.
Subminiature circuit breakers are used, for example, in aircraft electrical systems where they not only provide overcurrent protection but also serve as switches for turning equipment on and off. A circuit breaker push-pull handle is moved from in-to-out in order to open the load circuit. This action may be either manual or, else, automatic in the event of an overload or fault condition. If the push-pull handle is moved from out-to-in, then the load circuit is re-energized. If the load circuit had been automatically de-energized, then the out-to-in operation of the push-pull handle corresponds to a circuit breaker reset action.
Typically, subminiature circuit breakers have only provided protection against persistent overcurrents implemented by a latch triggered by a bimetal responsive to I2R heating resulting from the overcurrent. There is a growing interest in providing additional protection, and most importantly arc fault protection. Arc faults are typically high impedance faults and can be intermittent. Nevertheless, such arc faults can result in a fire. During sporadic arcing fault conditions, the overload capability of the circuit breaker will not function since the root-mean-squared (RMS) value of the fault current is too small to activate the automatic trip circuit. An electronic arc fault sensing circuit directly trips and, thus, opens the circuit breaker.
Although many circuit breakers also employ ground fault protection, in aircraft applications, the aircraft frame is ground, and there is no neutral conductor. Some aircraft systems have also provided ground fault protection, but through the use of additional devices, namely current transformers which in some cases are remotely located from the protective relay.
Electronic circuitry may malfunction if its alternating current (AC) power source is operating below the minimum voltage and/or frequency levels necessary to maintain the stability of such circuitry""s direct current (DC) regulated power supply or supplies.
For example, an aerospace arc fault circuit breaker may employ an analog electronic circuit to implement an envelope arc detection algorithm. See U.S. Pat. No. 5,818,237. The analog electronic circuit utilizes a dual voltage (positive and negative 13.6 VDC) power supply, which is derived from a 120 VAC, 400 Hz aircraft power line by a capacitively coupled voltage dropping network. As is common in this type of power supply, the two 13.6 VDC power supplies lose regulation if the 120 VAC source voltage and/or 400 Hz frequency fall low enough such that the current flowing through the voltage dropping network is less than what is consumed by the circuit itself. Once regulation is lost, the envelope arc fault detection circuit may malfunction and generate a false output indication, thereby causing the circuit breaker to trip open. This unwanted tripping of the circuit breaker is unacceptable since it could interrupt the operation of a flight critical system being sourced by the circuit breaker and, thus, compromise safe operation of the aircraft.
There is room for improvement in circuit breakers.
The present invention is directed to a circuit breaker, which employs a power supply monitor circuit for a power supply having a first regulated direct current voltage with a first polarity and a second direct current voltage with a second polarity, which is opposite the first polarity. The monitor circuit has an output, which disables a circuit breaker trip mechanism, which is powered from the first direct current voltage.
In accordance with the invention, a circuit breaker comprises: separable contacts; an operating mechanism for opening and closing the separable contacts; a power supply comprising an input adapted to receive an alternating current voltage, a regulator adapted to regulate a first direct current voltage having a first polarity, a first output adapted to provide the first direct current voltage, and a second output adapted to provide a second direct current voltage having a second polarity which is opposite the first polarity; a trip mechanism cooperating with the operating mechanism, powered from the first output of the power supply, and responsive to electrical conditions of the separable contacts for tripping the separable contacts open, the trip mechanism having an input for disabling the tripping; and a power supply monitor circuit comprising: a node adapted for electrical connection to a ground, a first resistor electrically connected in series with the regulator of the power supply, a first transistor having two inputs electrically connected in parallel with the first resistor and having an output, a first capacitor electrically connected between the first output of the power supply and the output of the first transistor, a second resistor electrically connected in series with the first capacitor between the first and second outputs of the power supply, a second transistor having a first input electrically interconnected with the output of the first transistor, a second input and an output, a diode electrically connected between the node and the second input of the second transistor, and an interface between the output of the second transistor and the input of the trip mechanism.
Preferably, the interface includes a transistor having a collector, which is open when the first output of the power supply is in regulation; and the trip mechanism includes an arc fault detector powered from the first output of the power supply, the arc fault detector has a capacitor, which is electrically connected to the collector, and which is discharged by the collector when the power supply is out of regulation in order to disable the arc fault detector.